A radiation image recording and reproducing method utilizing a stimulable phosphor described, for instance, in U.S. Pat. No. 4,239,968, is now practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of radiographic film and radiographic intensifying screen. Further, the radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis.
The radiation image storage panel employed in the above-described method comprises a stimulable phosphor layer which is optionally provided on a support. Further, a transparent layer of a polymer material is generally provided on the free surface (e.g., surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor layer generally comprises a binder and a stimulable phosphor (in the form of particles). The stimulable phosphor emits light (that is, gives stimulated emission) when it is excited with an electromagnetic wave (i.e., stimulating rays) after it is exposed to radiation such as X-rays. In more detail, the radiation having passed through an object or radiated from an object is absorbed by the stimulable phosphor layer of the panel in an amount proportional to the applied radiation dose, and a radiation image of the object is formed on the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the storage panel with stimulating rays. The stimulated emission is then photoelectrically detected to give a series of electric signals, so as to reproduce a visible image from the electric signals.
In the radiation image recording and reproducing method, radiation image storage panels of other types such as those having a vacuum-deposited or sintered stimulable phosphor layer may be employed.
The radiation image recording and reproducing method is generally performed in a united radiation image recording and reading apparatus which comprises recording means (for applying a radiation having an image information to the radiation image storage panel to record the radiation image on the storage panel); reading means (for irradiating the stimulating rays to the storage panel having the radiation image to produce stimulated emission from the storage panel and photoelectrically reading the stimulated emission); erasing means (for applying an erasing light to the storage panel after the reading step is complete to remove a radiation image remaining in the storage panel); and transfer system (which is arranged between these means, for transferring the storage panel from one means to another means in predetermined order). Alternatively, the radiation image recording and reading apparatus may comprise two separated apparatuses, that is, a radiation image recording apparatus and a radiation image reading apparatus equipped with erasing means.
In any of the radiation image recording and reproducing systems, the radiation image storage panel is repeatedly employed after the remaining radiation image is erased.
In the radiation image recording and reproducing method, the radiation image recorded in the storage panel is generally read by applying the stimulating rays to one side of the storage panel and collecting light emitted by the phosphor particles by means of a light-collecting means from the same side (hereinafter referred to as "single-side reading system"). There is a case, however, that the light emitted by the phosphor particles should be collected on both sides of the storage panel. This is because the emitted light is desired to be collected as much as possible. There also is a case that the radiation image recorded in the phosphor layer varies along the depth direction of the layer and such variation of the radiation image should be detected. An example of the system for reading the radiation image from both sides (hereinafter referred to as "double-side reading system") is illustrated in FIG. 1 of the attached drawings.
In FIG. 1, the radiation image storage panel 11 is transferred (or moved) by a combination of two sets of nip rolls 12a, 12b. The stimulating rays such as laser beam 13 is applied to the storage panel 11 on one side, and the light emitted by the phosphor particles advances upward and downward (in other words, toward both the upper and lower surfaces). The downward light 14a is collected by a light collector 15a (arranged on the lower side), converted into an electric signal in a photoelectric conversion device (e.g., photomultiplier) 16a, multiplied in a multiplier 17a, and then sent to a signal processor 18. On the other hand, the upward light 14b is directly, or after reflection on a mirror 19, collected by a light collector 15b (arranged on the upper side), converted into an electric signal in a photoelectric conversion device (e.g., photomultiplier) 16b, multiplied in a multiplier 17b, and then sent to the signal processor 18. In the signal processor 18, the electric signals sent from the multipliers 17a, 17b are processed in a predetermined manner such as addition processing or reduction processing depending upon characteristics of the desired radiation image.
The radiation image storage panel 11 continuously advances in the direction indicated by the allow by means of the nip rolls 12a, 12b. Accordingly, the area of the storage panel which is subjected to the stimulating step (i.e., reading step) is subjected to an erasing step which uses an erasing lamp 20 such as a sodium lamp. In the erasing step, the radiation energy which still remains in the storage panel after being subjected to the reading step is almost completely released from the storage panel. Therefore, the radiation image storage panel having been subjected to the erasing step contains almost no latent image composed of the remaining radiation energy, and is favorably employed in the next cycle of the radiation image recording and reproducing method.
According to studies of the present invention on the radiation image reading system illustrated in FIG. 1 which comprises the successive stimulating and erasing steps applied in parallel on the same storage panel, it has been noted that the radiation image in that system sometimes suffers from noise. Therefore, the inventor has further studied the reason of noise observed in the reproduced radiation image, and discovered that the noise is produced by a portion of the erasing light which is applied on the adjacent area of the storage panel and then moves in a zig-zag mode in the horizontal direction within the transparent support of the storage panel along its plane to reach the area which is under stimulation.
The troublesome noise can be removed by not performing the reading step and the erasing step on the storage panel in parallel. However, the procedures of the combined reading and erasing step in parallel are very advantageous from the viewpoint of performing the radiation image recording and reproducing method quickly and efficiently. Such system is also advantageous because the whole apparatus can be constructed in a relatively small size.
U.S. Pat. No. 5,534,710 describes the use of a radiation image storage panel comprising a stimulable phosphor layer and an erasing light-diffusion preventive layer capable of allowing transmission of stimulated emission while preventing an erasing light impinged thereupon from diffusing within the preventive layer along the plane thereof in the radiation image reading system employing the double-side reading system which comprises the steps of exciting an area of the phosphor layer of the radiation image storage panel which has a radiation energy therein in the form of a latent image with stimulating rays to release the radiation energy from the phosphor layer as stimulated emission and simultaneously detecting photoelectrically the stimulated emission from both sides of the storage panel to obtain electric signals for reproduction of a radiation image; and applying an erasing light on the same area of the radiation image storage panel after completion of the above exciting and detecting step in parallel with a successive exciting and detecting step applied simultaneously on other area of the storage panel. The U.S. patent further describes that the erasing light-diffusion preventive layer of the above radiation image storage panel may be in the form of a colored layer capable of absorbing the erasing light, a layer diffuses the erasing light on the interface between said preventive layer and the stimulable phosphor layer, or an optical path-limiting layer capable of accepting only a light impinged thereupon at an incident angle of 30.degree. or more and guiding the accepted light to pass therethrough vertically.